The cowrie shell (*Cypraea moneta*), bred almost exclusively in the Maldives and shipped to West Africa as ballast on East India Company vessels from the 16th century, was the principal currency of the Bight of Benin, the Bight of Biafra, the Yoruba towns and the Sokoto Caliphate for roughly three centuries. Marion Johnson (*The Cowrie Currencies of West Africa*, JAH 1970) estimated cumulative 19th-century imports into West Africa at over 10 billion shells, the bulk landing through Lagos, Badagry, Whydah, Bonny, Brass and Calabar. A prime slave at Bonny in the 1780s cost 12,000–16,000 cowries ('one head'); a tin of palm oil in the 1850s, 2,000.
The collapse was engineered in three converging acts.
1. The Zanzibar dump (1845–1870s). British and German firms opened a new supply of the larger East African ring cowrie (*Cypraea annulus*) from Zanzibar and Mozambique and shipped it into Lagos and the Oil Rivers in industrial tonnages. Hopkins (1966) and Ofonagoro (1979) document a Lagos cowrie-to-sterling collapse of over 80% between 1851 and 1869 — a textbook colonial-induced hyperinflation that wiped out the working capital of the coastal merchant houses (Kosoko, Pepple, Jaja, Nana, Ovonramwen) precisely as Britain was annexing the same coast.
2. Legal demonetisation (1880–1908). On the basis of the Lagos Currency Proclamation of 1880, the British silver shilling was declared legal tender in Lagos Colony at a fixed conversion that the market refused to honour. The Native Revenue Proclamation of 1906 and the Native Revenue Ordinance of 1908 (extended progressively to Southern Nigeria) required direct tax — Lugard's signature instrument of indirect rule — to be paid in coin, not in cowries or manillas, forcing every peasant farmer into the cash-crop economy to acquire sterling. Cowries were demonetised for tax in Southern Nigeria by 1908; in the North, the parallel cowrie–Maria Theresa thaler economy resisted until the 1911 tax reforms that triggered, among other revolts, the Satiru Rising of 1906 and persistent emirate-level non-compliance into the 1920s.
3. WACB monopoly (1912). The West African Currency Board, established by Colonial Office order on 6 November 1912 and headquartered in London with a regional office in Lagos, issued sterling-parity WACB notes and coins for the four British West African territories and bought out the remaining indigenous currencies at a loss to their holders. Manillas — the horseshoe-shaped brass currency of the eastern Delta and Igboland — were tolerated until Operation Manilla (1948–49), when the colonial government withdrew 32 million pieces from circulation at a fixed buy-back price; the operation was the largest single act of monetary demonetisation in West African history before the 1984 currency change.
The conquest mechanism. The crash of indigenous currency value (1) collapsed the working capital of the coastal merchant princes who would otherwise have resisted treaty terms — explaining the relative ease with which Jaja of Opobo (deported 1887), Nana Olomu of Itsekiri (Ebrohimi sacked 1894, deported to Accra), Oba Ovonramwen of Benin (Punitive Expedition 1897) and the Aro Confederacy (Anglo-Aro War 1901–02) were broken. The crash also (2) created the tariff revenue that funded the conquest itself: Royal Niger Company tariffs on the Niger were payable in sterling, generating the surplus that paid Lugard's West African Frontier Force. And (3) the tax-in-coin requirement of 1908 compelled every household in Southern Nigeria to enter the colonial labour market — the foundation of the cash-crop colonial economy that fed Liverpool and Manchester until independence.
The one-line summary, defensible from the documentary record: the Maxim gun arrived in Nigeria after the currency had already been broken.